1. Field of the Invention
The present invention relates to an electrode wire for use in electrical discharge machining and a method of manufacturing the same, particularly to a porous electrode wire having an improved machining speed and a method of manufacturing the same.
2. Description of the Prior Art
FIG. 1 represents a schematic drawing of a wire electrical discharge machine. An electrode wire is inserted through a start hole (7) of a workpiece (1), which is continuously fed through the hole. A high frequency voltage is applied between the wire (2) and the inside of the hole (7) to initiate an arc discharge between them. Then, machining the workpiece (1) to a desired shape can be achieved by melting the workpiece during the arc discharge and by removing the machining particles using a machining liquid and an instantaneous vaporization power between the wire and the workpiece. In accordance with the machining principle, the wire electrical discharge machine includes a power supply, a wire transfer means, a workpiece moving means and a circulating means for circulating the machining liquid.
In general, the workpiece moving means, as indicated by the arrow in FIG. 1, moves during the machining of a workpiece on a plane perpendicular to the wire feeding direction. The wire (2) from a supply spool (3) travels to a take-up roll (4) through a wire transfer means including the upper and the lower guide rollers (5 and 5xe2x80x2) of the workpiece.
Then, a high frequency voltage (6) is applied between the workpiece (1) and the electrode wire (2) to start the machining of the workpiece. At the same time, a machining liquid of deionized water is supplied to the machining area to remove the heat resulting from the machining. The machining efficiency, in particular the machining speed, significantly depends on machining parameters such as the feeding speed of the machining liquid, machining current, and the shape and frequency of the machining voltage, and it is known to improve the machining efficiency through control of the machining parameters.
Since copper has a high electrical conductivity and is easy to form fine wire from, due to its high elongation property, a copper wire was used initially. However, it revealed many deficiencies mainly due to its low mechanical strength. For example, high tensile strength could not be applied to the copper wire during the machining so that vibration of the wire cannot be controlled, resulting in an inferior machining accuracy and a tendency of wire breakage. Moreover, machining speed was relatively slow. Therefore, a molybdemum wire or a tungsten wire as a high strength wire has been used for a special application of a high precision machining. A brass wire having 63-67 wt % of copper and 33-37 wt % zinc has been developed for the general purpose of wire electrical discharge machining.
The brass wire has a tensile strength of about twice that of copper wire and the machining speed can be improved due to the presence of zinc content in the alloy, which provides a stable discharge and a vaporization power during the machining.
Moreover, as the application field for the wire electrical discharge expanded, further increase in the tensile strength of a brass wire was required in order to improve the machining speed. Therefore, elements such as A1 and/or Si can be added to a brass wire to improve the tensile strength and machining speed.
On the other hand, it was known that the machining speed of a brass wire increases when zinc content includes more than 40 wt % in the brass. However, in that case, the drawing process to form a wire becomes difficult because of the presence of a brittle phase in the alloy.
U.S. Pat. No. 4,287,604 discloses an zinc coated wire on copper or brass core and the method of manufacturing the same. On a core material having relatively high tensile strength or high electrical conductivity such as copper, brass or steel, a coating material having a relatively low vaporization temperature such as zinc, cadmium, tin, antimony, bismuth or an alloy thereof was electroplated to form a coated wire. According to the wire and the method, the core enables the wire to maintain required mechanical strength or conductivity, and the coating increases cooling ability and flushability because of its relatively low vaporization temperature, thereby improving machining speed and accuracy. Further, the coating material vaporizes easily by the heat generated during the machining, and it protects the core material because of the cooling effect of the coating material. Thus, the method of manufacturing the coated wire may include the coating step of zinc electroplating after the final sizing of the wire, or prior to the final sizing of the wire.
A method of improving the performance of a coated wire was disclosed in U.S. Pat. No. 4,977,303. According to the patent, the method includes steps of; on a metallic core, a coating step of zinc, cadmium or the alloy which which forms mixed alloy layer with the core after heat treatment by diffusion annealing; a heat treatment step of the coating wire at 700xc2x0 C. in an oxidizing atmosphere to form a mixed alloy layer between the core material and the coating material, for example copper-zinc alloy and drawing the coated wire accompanying a mechanical hardening. The coated wire formed by that method includes a core, a mixed alloy layer and an outer oxide layer. At this time, the oxide layer prevents the possibility of short circuits between the wire and the workpiece during the electrical discharge machining, which is not directly related to the machining speed. The improvement in the machining speed of the wire is known to result from the heat treatment step forming copper-zinc alloy layer, but the mechanism was not clearly revealed.
U.S. Pat. No. 4,686,153 discloses a coated wire having a copper clad steel core and a coating layer of zinc alloy formed on the core, and the method of manufacturing the same. The high strength of steel in the core can provide a superior machining accuracy and the clad copper can provide good conductivity to the coated wire. On this copper clad steel, zinc coating is applied by electroplating or hot dip galvanizing followed by heat treatment to form a copper-zinc alloy layer. Particularly, when the zinc content in the alloy layer is in the range of 40-50 wt %, the improvement of the coated wire in machining speed becomes evident compared with a simple zinc coated copper clad steel. The coated wire according the patent includes a copper clad steel core and a copper-zinc alloy layer. At the same time, the zinc content in the alloy layer ranges 10-50 wt %, preferably 40-50 wt %. The method of manufacturing the same includes steps of; a providing step of a copper clad steel core, a zinc electroplating step on the core, a drawing step of the zinc coated core to form a wire having a desirable diameter and a heat treatment step of the wire to convert the zinc coating layer into a copper zinc alloy layer having zinc content of 10-50 wt %, preferably 40-50 wt % in such a manner that the concentration of the zinc is gradually decreased along the radially inward direction. Alternatively, the drawing step may be applied prior to the heat treatment step and the zinc coating may use hot dip galvanizing.
As mentioned previously, the improvement of the machining speed of the coated wire was achieved by coating the core with a material such as zinc, which has a melting temperature and vaporization temperature lower than the core material, and a further improvement was achieved by heat treating the zinc layer on the core to form a copper-zinc alloy layer through a diffusion reaction between the core and the coating. However, the improvement significantly depends on the selection of a coating metal having lower vaporization temperature than the core metal. Thus, the improvement was limited to the nature of the coating metal.
A purpose of the present invention is to provide a coated wire for electrical discharge machining with improved machining speed by increasing the surface area of the wire which will be in contact with cooling liquid so as to increase the cooling ability of the wire.
Another purpose of the invention is to provide a coated wire for electrical discharge machining with improved machining speed by allowing the contact of the cooling liquid not only with the surface of the wire, but also with inner part of the wire.
Still another purpose of the invention is to provide a method of manufacturing a porous coated wire with increased surface area without additional steps.
Still another purpose of the invention is to provide a coated wire for electrical discharge machining with improved flushability without decreasing the machining accuracy during the machining.
Therefore, the above-mentioned purposes are achieved by a method including the steps of; providing a wire having a first diameter made of a first metal, hot dip galanizing the wire by passing the wire in for a desirable time through a molten bath of a second metal having a vaporization temperature lower than the first metal, thereby forming an alloy layer by the diffusion reaction between the first metal and the second metal, the second metal having hardness higher and an elongation lower than the first metal and the second metal and a coating layer made of the second metal, and drawing the wire having the alloy layer and the coating layer to form a second diameter, thereby forming cracks in the alloy layer and the coating layer due to the high hardness and the low elongation of the alloy layer.
At this time, the first metal may use copper or brass having 63-67 wt % copper and 33-37 wt % zinc. Further, the second metal may use zinc, aluminum or tin.
Particularly in the present invention, the wire made of the first metal needs to pass through the molten bath within a desirable time frame so as to achieve a desirable thickness of the coating layer and alloy layer including the second metal. The desirable time depends on the length of the bath. For example, the take-up speed of the wire should be fast when the length of the bath is relatively long, and the take-up speed of the wire should be slow when the length of the bath is relatively short. Thus, the take-up speed and the length of the bath are selected to form the thickness of the coating layer having 3-10 xcexcm on the wire having the first diameter.
The method of manufacturing a coated wire according the present invention may further include a heat treatment step to stabilize the mechanical property of the wire.
Further, the method may include a removing step for removing the coating layer on the alloy layer.
The coated wire for electrical discharge machining, according to the present invention, includes a core made of a first metal including copper, an alloy layer formed on the core and a coating layer made of a second metal, wherein the alloy layer having the highest hardness among the core and coating layer is formed during the hot dipping galvanizing step by a diffusion reaction between the first metal and the second metal, the second metal having vaporization temperature lower than the first metal, and wherein the alloy layer includes cracks having a direction perpendicular to the longitudinal direction of the wire.
The alloy layer having a high hardness and a low elongation is formed by a diffusion reaction between the first metal and the second metal during the hot dip galvanizing step passing the wire, having the first diameter, made of the first metal into a molten bath of the second metal having lower vaporization temperature. Then, the wire coated with the second metal is drawn to form the wire having the second diameter. At this time, the first metal covered by the alloy layer and the coating layer becomes a core of the wire. Further, the porous nature of the wire arises from the cracks in the alloy layer and the coating layer during the drawing step.
Further, the coated wire for electrical discharge machining, according to the present invention, includes a core made of a first metal including copper and an alloy layer formed on the core, wherein the alloy layer having a higher hardness than the core and coating layer, is formed during the hot dipping galvanizing step by a diffusion reaction between the first metal and the second metal, the second metal having a vaporization temperature lower than the first metal, and wherein the alloy layer includes cracks having a direction perpendicular to the longitudinal direction of the wire.